| The electromagnetic wave propagation under the coal mine is complex which may influence the communication performance of mobile users in underground coal mine. But the underground space, time and frequency resources are open. The coal mine MC-CDMA (Multiple Carrier-Code Division Multiple Access) wireless transmission can make full use of these open resources to improve the wireless transmission performance. Based on the time-frequency coded cooperation MC-CDMA modulation, this paper proposed adaptive power allocation, multi-user detection and joint cross-layer transmission algorithms to improve the wireless communication performance of users.When user takes MC-CDMA modulation, different sub-channel of the user has different channel gain, which will lead to different receive SNR (Signal Noise Ratio) at each sub-carrier. This paper uses SNR at the transmitter in the simulations to study the BER (bit error rate) of the user. In order to focus on the performance of the algorithms at high BER, the simulations are carried out at the range (0,0.5] of BER. The results show that the performance of the users at high BER range under the proposed algorithms is improved obviously.(1) In order to make full use of the limited power of coal mine, this paper researched adaptive power allocation of coded cooperation MC-CDMA. The sum of transmission power of the target user and cooperative user is a fixed value in the system. As the channel gains among target user, cooperative user and base station vary with the the location of users, the equal power allocation scheme between target user and cooperative user may result in the waste of power resource. In order to make full use of the limited power and ensure the performance of wireless communication of target users, a SNR (signal-to-noise ratio) threshold at the target user, cooperative user and base station is set up. It should be guaranteed that the received SNRs at the target user, cooperative user and base station are larger than the threshold which is the power of target user to total power. The range of power distribution ratio for the time-frequency coded cooperation MC-CDMA is deduced and the median value of the range is taken as the ratio between target user and cooperative user. Since the power distribution ratio varies with the total power and average channel gain, the power of target user and cooperative user can be allocated dynamically to achieve the adaptive allocation power.(2) Aiming at the problem of the interference between multi-users in MC-CDMA wireless transmission system under mine tunnel, the multi-user detection algorithm based on particle swarm optimization was proposed to reduce or eliminate the interference between multiple users. The proposed particle swarm multi-user detection () uses particle swarm optimization algorithm at the cooperative user and the base station to optimize the output result of the decorrelation multiuser detection (D-MUD). The performance of the proposed D-PSO multiuser detection algorithm is better than the D-MUD and the complexity is far below the optimum multiuser detection (O-MUD). Simulation results show that the solution improved the performance of time-frequency coded cooperation MC-CDMA wireless transmission network significantly with relatively low complexity.(3) Aiming at the problem that the high Peak to Average Power Ratio (PAPR) of MC-CDMA can cause nonlinear distortion of the transmitted signal and the performance of the underground wireless network, a phase compensation algorithm based on Selective Mapping (SLM) was proposed to suppress the PAPR. In order to further improve the monitoring performance of the underground wireless communication system, a joint cross-layer design scheme based on time-frequency coded cooperative Hybird Automatic Repeat reQuest (HARQ) was proposed, which is the combination of time-frequency coded cooperation of physical layer and HARQ of MAC layer. Cooperative user takes a time-frequency coding before retransmitting the packet of target user to obtain the coding and diversity gain. The simulation results show that the coal mine adopting the joint cross-layer transmission of time-frequency coded cooperative HARQ based on SLM phase compensation algorithm can reduce PAPR of MC-CDMA modulation and improve the performance of MC-CDMA wireless transmission network under coal mine.Above algorithms applied to the time-frequency coded cooperation MC-CDMA wireless transmission system can effectively improve the performance of target user with poor channel condition and improve the bit error rate(BER) performance and capacity of underground wireless transmission network. These algorithms are very conducive to the sustainable production under coal mine. |
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